The forming and processing of radiographically produced shadowgraphs or radiation transmission patterns to produce visual images of a specimen or workpiece is of interest in various applications, such as the radiographic inspection of various structural components. Previously, such inspection techniques entailed the forming of photoshadowgraphs. A photographic film plate was positioned adjacent an object to be inspected by the neutron or X-ray source, the object being positioned between the film and the source of radiation. When neutron or X-radiation is transmitted through any hetrogeneous object, it is differentially absorbed, depending upon the varying thickness, density, and chemical composition of the object. The image registered by the emergent rays on a film adjacent to the specimen under examination constitutes a shadowgraph, or radiograph, i.e. an intensity pattern of the rays transmitted, of the interior of the specimen.
X-radiation may be used in industrial applications wherein, for example, it is desired to evaluate a metal casting suspected of having internal cracks, separations, voids, or other defects; and it is, of course, employed widely in medical applications. X-rays are, in general, substantially more penetrating than neutron radiation with respect to "low-z" materials such as aluminum, plastic, boron, carbon, and the like. Radiographs produced from neutron radiation are employed, for example, when it is desired to form an image of hydrogenous, or organic materials which may be present within metallic structures. Neutrons penetrate low-thermal-cross-section materials such as lead, aluminum, steel, and titanium, but are absorbed by organic, hydrogenous materials. With respect to metallic stuctural members, an indication of such hydrogenous materials within the structure may reveal the presence of water, hydroxides, and other corrosion products. Such corrosion may be in the form of intergranular corrosion, with accompanying exfoliation, of materials such as aluminum, and certain other metals. Stresses in aluminum aircraft components, for example, produce internal, intergranular corrosion which is invisible and not accurately imaged by conventional, non-destructive inspection techniques; such corrosion may result in critical failure of major structural elements if it continues undetected. As in the design of load-bearing or structural members for various industrial applications, the conventional design philosophy for aerospace components entails a substantial degree of "over design" for ensuring structural integrity of the components. As will be understood by those in the art, such an excess of material results in correspondingly higher weight and cost, and in lower performance and fuel efficiency than would be obtained if compensation for potential, undetectable internal deterioration was not necessary. Similarly, the permissible useful life of such components is also based upon safety margins which can be substantially reduced if positive assurance were obtainable that internal, or hidden deterioration had not occurred to a significant degree.
Further difficulties with respect to non-destructive testing of aerospace components relate to the possibilities of surface corrosion on internal components hidden from visual inspection. Corrosion which may occur within honeycomb cell tructures or panels may result in the separation of honeycomb cores from outer skin surfaces, and the like.
In the past it has been attempted to produce images produced from low level radiation such as neutron, or low level X-radiation, by exposing photographic films to the radiation for an appropriate period of time and developing the film for inspection. The use of photographic film provides the advantage that, through exposure over an extended period of time, very low levels of radiation may form a satisfactory photoradiograph. Exposure times, film speed, radiation levels and film types may be varied. It will be understood, however, that the delays entailed in set-up and film processing imparts limitations in inspection efficiency, particularly when it is desired to inspect, and reinspect, large components or large numbers of components. For this reason, modern radiographic inspection systems have employed low-light-level television cameras for producing television images derived from the radiation of a specimen, whereby a television display corresponding to a radiophotograph is formed. The television monitor may be located in a facility remote from the radiation source, which may afford advantages when hazardous radiation is present. Additionally, television monitoring permits continuous monitoring of a component for real, or "near real time" examination. Such low-light-level television cameras may be of the image orthicon type or of other types, and often employ multiple stages of image intensification or amplification. Modern, low-light-level cameras include various refinements and intensification techniques, such as silicon intensified targets (SIT), secondary electron conduction (SEC), charge-storing, and amplifying.
Two general approaches to the formation of television images of irradiated specimens are illustrated in U.S. Pat. Nos. 3,280,253 and 3,668,396, to R. C. McMaster et al and J. A. Asars et al, respectively, both of which are hereby incorporated by reference. The system of the McMaster patent employs a single stage camera tube which is sensitive to X-radiation. In use, a radiation source is positioned to direct X-radiation directly toward the television camera tube after transmission through a workpiece to be inspected, and an image is formed on the camera tube target by electrons derived from the X-radiation directed toward the camera; the image is intensified by the use of periodic beam scanning, in which the radiation builds up adequate image potential (an image pattern comprising a loss of positive charges at portions of a semiconductor target) between raster scanning cycles. A satisfactory TV image is produced by intermittent scanning of the target by the electron beam raster scanner. The McMaster camera includes no intermediate intensifying stages. Such single stage camera tubes provide relatively moderate gain in comparison with highly sensitive tubes such as that disclosed in the recent Asars patent. The McMaster system may thus be considered to have a relatively high level of input radiation (radiation directly from the X-ray source, which is of generally higher intensity and penetrating potential than portable neutron sources) and a relatively low level of internal intensification or amplification in comparison with multi-stage cameras such as that disclosed in the Asar patent. Such systems are advantageous for certain applications, and such single stage television cameras are less expensive and complex than multi-stage, very low-light-level cameras.
The Asars system employs a phosphor screen to provide a large field of view of appropriate resolution and detail, the phosphor screen serving to generate scintillations of light as the screen receives gamma radiation derived from a neutron source. The light scintillations on the screen are detected and intensified through the sensitive, multi-stage SEC camera tube. To provide adequate light amplification, the camera tube employs several stages of image intensification, including an initial image intensifier tube section and an intermediate image intensifying section. As will be understood by those in the art, sophisticated low-light level cameras such as that employed in the Asars system are highly complex and expensive.
The present system is intended to provide a radiographic television display with a relatively lower cost and less complex camera system, while at the same time providing very high sensitivity to low radiation levels. In particular, it is intended to provide a radiographic system sensitive to "soft" or thermal neutron radiation, i.e., radiation from which the higher energy neutron and gamma rays have been removed, as may be obtained from portable radiographic generator systems such as that disclosed in U.S. Pat. No. 4,300,054, issued Nov. 10, 1981, to W. E. Dance et al, which is incorporated by reference. The system of the U.S. Pat. No. 4,300,054 employs a modulator fluid and filter for attenuating the hard, gamma radiation from energy produced by a radiation generator tube. There is a need, particularly in the inspection of aircraft and other components by low power, non-isotopic radiation sources, for an efficient television radiographic display means wherein a high resolution image is produced for convenient viewing. In such systems, radioactive isotopes as radiographic sources are not employed, eliminating the hazards and inconveniences entailed in the transportation and storage of such materials.
A problem entailed in prior radiographic systems has been difficulty in producing a high resolution, finely detailed image in the presence of varying levels of radiation. High radiation peaks may tend to overload and blur the camera and may even damage the camera. Another problem has been that very low levels of radiation, such as those obtained from thermal neutron sources and from low level X-rays, have been difficult to record because of inherent system noise. The obtaining of detailed images required to show fissures and details of internal deterioration of metals with sufficient resolution to ensure that no critical faults exist in a piece under inspection is of importance in many applications. A further deficiency in prior inspection systems has been their limitation to undesirably narrow ranges of energy levels. That is, those instruments sensitive to high level radiation such as that produced by X-radiation have been insensitive and not usable with lower levels of radiation commonly received as neutron radiation. Prior systems were not usable with low-level neutron radiation.